The invention relates to a method of manufacturing a disc resonator consisting of a single crystal of a magnetic garnet material having the composition Y.sub.3 (Fe.sub.2-x M.sub.x)(Fe.sub.3-y M.sub.y)O.sub.12, where M is a nonmagnetic ion which can occupy both octahedral and tetrahedral lattice sites. The method is performed by providing a single crystal of the magnetic garnet material, and then cutting the single crystal along a given crystal direction into thin discs. The thickness-to-diameter ratio of each disc is less than approximately 0.3. Finally, the discs are annealed.
As compared with conventional spherical resonators (such as ferrimagnetic resonators used for tunable oscillators and filters in the VHF, UHF and SHF frequency ranges), disc resonators of monocrystalline yttrium-iron garnet have a few advantages in particular when gallium, aluminium or another nonmagnetic ion is substituted at both octahedral and tetrahedral lattice sites. In principle, axially magnetized disc resonators can be operated at comparatively lower resonant frequencies than spherical resonators manufactured from the same material. Furthermore, there is no coupling between ferrimagnetic resonance and degenerated magnetostatic resonance modes in the disc resonators; in spherical resonators this effect presents serious problems below approximately 1.5 GHz.
A disadvantage of YIG disc resonators, however, is the strong temperature dependence of the resonant frequency. In spherical resonators, the temperature dependence of the resonant frequency, f.sub.r, can be adjusted, by choosing a suitable crystallographic orientation, to values of approximately zero for a given operating frequency range. This adjustment procedure cannot be performed on a disc resonator after the disc has been cut from the starting crystal because the resonance field, H.sub.Z, must be aligned parallel to the axis of the disc. The initial choice of the sectional plane from the starting Ga:YIG crystal determines at what temperature the sign of the contribution to the temperature dependence by the anisotropy field is opposite to the sign of the contribution to the temperature dependence of the demagnetization. In Ga:YIG this is a result of the negative crystal anisotropy when the axis of the disc is along the [111] direction of the crystal. Consequently a disc resonator of this material has a substantially temperature-independent resonant frequency within a fixed (i.e. nonadjustable) temperature range.
In principle the value of the resonant frequency compensation temperature, T.sub.o, of a disc resonator made from a given starting crystal can be influenced to a certain extent by choice of the ratio of the thickness, d, to the diameter, D, of the disc. However, in practice this measure can only be used to a limited extent, because both the thickness and the diameter of the disc are the main factors which determine the high frequency coupling of the disc resonator in the circuit in which it is used.
In components which include ferrimagnetic resonators, the permitted temperature behavior of the resonant frequency in the operating frequency range is fixed by various system requirements. As a result of this, the value of the total Ga substitution in a disc resonator depends upon the specification for T.sub.o and the d/D ratio; typically the Ga content must remain within a narrow range. This latter requirement for obtaining a desired T.sub.o is difficult to fulfil in practice because according to the known crystal growth methods for Ga:YIG, the Ga content can be predetermined only within a margin of approximately .+-.10%. As a consequence of the uncertainty of the Ga content, T.sub.o may vary by .+-.40.degree. C.